fix(#94): replace text-white with text-primary-fg on all primary buttons

In dark mode --c-primary switches from navy (#012851) to mint (#a1dcd8).
Buttons using bg-primary+text-white showed white text on mint at 1.4:1
contrast — invisible. bg-brand-navy buttons were also invisible (navy on
near-black canvas, 1.3:1).

Replaced in 28 components app-wide:
- bg-primary ... text-white → text-primary-fg
- hover:bg-primary hover:text-white → hover:text-primary-fg
- bg-brand-navy ... text-white + hover:bg-brand-navy/90 →
  bg-primary ... text-primary-fg + hover:bg-primary/90

Light mode is unchanged: primary-fg = white in light mode.
Dark mode: primary-fg = navy (#012851) on mint bg = readable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/admin/users/[id]/+page.svelte

@@ -85,7 +85,7 @@ const selectedGroupIds = $derived(data.editUser.groups?.map((g: { id: string })
 			</a>
 			<button
 				type="submit"
-				class="rounded-sm bg-primary px-5 py-2 font-sans text-xs font-bold tracking-widest text-white uppercase transition-opacity hover:opacity-80"
+				class="rounded-sm bg-primary px-5 py-2 font-sans text-xs font-bold tracking-widest text-primary-fg uppercase transition-opacity hover:opacity-80"
 			>
 				{m.btn_save()}
 			</button>

frontend/src/routes/admin/users/new/+page.svelte

@@ -61,7 +61,7 @@ let { data, form } = $props();
 				</a>
 				<button
 					type="submit"
-					class="rounded-sm bg-primary px-5 py-2 font-sans text-xs font-bold tracking-widest text-white uppercase transition-opacity hover:opacity-80"
+					class="rounded-sm bg-primary px-5 py-2 font-sans text-xs font-bold tracking-widest text-primary-fg uppercase transition-opacity hover:opacity-80"
 				>
 					{m.btn_create()}
 				</button>